Unraveling non-covalent interactions within flexible biomolecules: from electron density topology to gas phase spectroscopy

The NCI (Non-Covalent Interactions) method, a recently-developed theoretical strategy to visualize weak non-covalent interactions from the topological analysis of the electron density and of its reduced gradient, is applied in the present paper to document intra- and inter-molecular interactions in flexible molecules and systems of biological interest in combination with IR spectroscopy. We first describe the conditions of application of the NCI method to the specific case of intramolecular interactions. Then we apply it to a series of stable conformations of isolated molecules as an interpretative technique to decipher the different physical interactions at play in these systems. Examples are chosen among neutral molecular systems exhibiting a large diversity of interactions, for which an extensive spectroscopic characterization under gas-phase isolation conditions has been obtained using state-of-the-art conformer-specific experimental techniques. The interactions presently documented range from weak intra-molecular H-bonds in simple amino-alcohols, to more complex patterns, with interactions of various strengths in model peptides, as well as in chiral bimolecular systems, where invaluable hints for the understanding of chiral recognition are revealed. We also provide a detailed technical appendix, which discusses the choices of cut-offs as well as the applicability of the NCI analysis to specific constrained systems, where local effects require attention. Finally, the NCI technique provides IR spectroscopists with an elegant visualization of the interactions that potentially impact their vibrational probes, namely the OH and NH stretching motions. This contribution illustrates the power and the conditions of use of the NCI technique, with the aim of providing an easy tool for all chemists, experimentalists and theoreticians, for the visualization and characterization of the interactions shaping complex molecular systems.

Gas-phase folding of a two-residue model peptide chain: on the importance of an interplay between experiment and theory

In order to assess the ability of theory to describe properly the dispersive interactions that are ubiquitous in peptide and protein systems, an isolated short peptide chain has been studied using both gas-phase laser spectroscopy and quantum chemistry. The experimentally observed coexistence of an extended form and a folded form in the supersonic expansion was found to result from comparable Gibbs free energies for the two species under the high-temperature conditions (< or = 320 K) resulting from the laser desorption technique used to vaporize the molecules. These data have been compared to results obtained using a series of quantum chemistry methods, including DFT, DFT-D, and post-Hartree-Fock methods, which give rise to a wide range of relative stabilities predicted for these two forms. The experimental observation was best reproduced by an empirically dispersion-corrected functional (B97-D) and a hybrid functional with a significant Hartree-Fock exchange term (M06-2X). In contrast, the popular post-Hartree-Fock method MP2, which is often used for benchmarking these systems, had to be discarded because of a very large basis-set superposition error. The applicability of the atomic counterpoise correction (ACP) is also discussed. This work also introduces the mandatory theoretical examination of experimental abundances. DeltaH(0 K) predictions are clearly not sufficient for discussion of folding, as the conformation inversion temperature is crucial to the conformation determination and requires taking into account thermodynamical corrections (DeltaG) in order to computationally isolate the most stable conformation.

Study of the docking of competitive inhibitors at a model of tyrosinase active site: insights from joint broken-symmetry/Spin-Flip DFT computations and ELF topological analysis

Following our previous study (Piquemal et al., New J. Chem., 2003, 27, 909), we present here a DFT study of the inhibition of the Tyrosinase enzyme. Broken-symmetry DFT computations are supplemented with Spin-Flip TD-DFT calculations, which, for the first time, are applied to such a dicopper enzyme. The chosen biomimetic model encompasses a dioxygen molecule, two Cu(II) cations, and six imidazole rings. The docking energy of a natural substrate, namely phenolate, together with those of several inhibitor and non-inhibitor compounds, are reported and show the ability of the model to rank the most potent inhibitors in agreement with experimental data. With respect to broken-symmetry calculations, the Spin-Flip TD-DFT approach reinforces the possibility for theory to point out potent inhibitors: the need for the deprotonation of the substrates, natural or inhibitors, is now clearly established. Moreover, Electron Localization Function (ELF) topological analysis computations are used to deeply track the particular electronic distribution of the Cu-O-Cu three-center bonds involved in the enzymatic Cu(2)O(2) metallic core (Piquemal and Pilmé, J. Mol. Struct.: Theochem, 2006, 77, 764). It is shown that such bonds exhibit very resilient out-of-plane density expansions that play a key role in docking interactions: their 3D-orientation could be the topological electronic signature of oxygen activation within such systems.

Lead substitution in synaptotagmin: a case study

Quantum chemistry computations have been used to investigate the possibility of a Pb(2+)/Ca(2+) substitution in the three calcium sites of the synaptotagmin enzyme. Provided explicit cation solvation is taken into account, it is shown that the substitution is energetically feasible and induces a strong reorganization of the Ca(2+)-coordinating sites, which may preclude the enzyme for any efficient role when lead poisoning occurs.